This invention relates to optical devices and, more particularly, to a reflective head-mounted see-through display that is light in weight and has excellent optical characteristics.
A head-mounted display system provides information to pilots and others so that they do not need to take their eyes from an external scene in order to obtain the additional information that is available for display. In one application, the head-mounted display system is mounted to the helmet of a pilot, and the display is projected on the surface of the front visor of the helmet in front of the pilot""s eyes. The pilot views the external scene directly, and also sees on the visor the display of additional information, such as a display of data or a secondary image such as an infrared image.
The helmet-mounted display requires that the display of additional information be projected at an angle from the side or top of the helmet and reflected from the curved visor to the eyes of the pilot. There is accordingly a significant distortion of the displayed information, the problem being somewhat like that of the driver of an automobile attempting to read a reflected image projected onto the windshield from the passenger seat. It is therefore necessary to pre-distort the image prior to its projection in a manner inverse to the distortion upon projection and reflection, so that the image viewed by the pilot has minimal distortion and is easily read.
In an existing helmet-mounted display such as that described in U.S. Pat. No. 5,499,139, a relay lens group projects the image produced by an image source toward the curved visor. The projection is made in a manner that pre-distorts the image so that the reflected display image ultimately viewed by the pilot has minimal distortion. The relay lens group of the ""139 patent has proved quite successful in accomplishing its optical objectives.
However, the approach of the ""139 patent leaves room for improvement in several areas. The relay lens group of the ""139 patent has two separate lens subgroups, which makes it relatively complicated, large in size, heavy, and expensive. The weight and size of the relay lens group is extremely important, because it is mounted to the helmet and thence must be supported by the head of the pilot. The relay lens group of the ""139 patent introduces a moment of inertia into the helmet structure that decreases the rate at which the pilot may turn the head, and also can become fatiguing on long missions. Because of its relative complexity, it is less robust in combat situations than is desirable. Lastly, it would be desirable to increase the pupil size of the projected display to make it more easily readable by the pilot and to allow the projected display to be optimally positioned.
There is, accordingly, a need for an improved head-mounted or helmet-mounted reflective display which improves the display system in the areas just discussed, while maintaining or improving upon its excellent optical performance. The present invention fulfills this need, and further provides related advantages.
The present invention provides a display device that may be used as a head-mounted display or a helmet-mounted reflective display. The display device has excellent optical performance with low distortion in the viewed image. It is light in weight and small in size, has a low moment of inertia mounted to the head or helmet, and is robust to meet safety and combat-environment requirements. The display device has a large pupil size and operates over a wide spectral band.
In accordance with the invention, a display device for a display wavelength range comprises an image source, a relay group made of optical elements transparent to the display wavelength range, and a reflective combiner in facing relation to the relay group. The relay group comprises an optical wedge having a front face in facing relation to the image source, and a back face; an aspheric lens module having a front face in facing relation to the back face of the optical wedge, and a back face; an aspheric lens having a front face in facing relation to the back face of the aspheric lens module, a back face, and an optical axis, the aspheric lens module being tilted and decentered with respect to the optical axis of the aspheric lens; a diffractive-optical-element lens module having a front face in facing relation to the back face of the aspheric lens, and a back face; and a positive-power lens module having a front face in facing relation to the back face of the diffractive-optical-element lens module, and a back face.
In a preferred approach, the image source is a miniature image source such as an active matrix liquid crystal display or a cathode ray tube, and the reflective combiner is part of a helmet visor. The optical wedge is made of a low-dispersion glass material. The aspheric lens module is made of plastic and comprises a positive-lens singlet made of a low-dispersion material, and a negative-lens singlet made of a high-dispersion material, and has its aspheric surface on the positive-lens singlet. The aspheric lens is made of plastic. The diffractive optical element lens is made of plastic and comprises a lens body, and a diffractive optical element embossed on the front face of the lens body with a grating spacing of greater than about 10 micrometers. The positive-power lens module is made of glass and comprises a positive-power singlet made of a low-dispersion glass material, and a negative-power singlet made of a high-dispersion glass material.
The use of optical-quality plastic elements where possible reduces the weight and cost of the display device. Compared with a conventional design, the weight of the relay group is reduced by about 60 percent, which in turn reduces the moment of inertia of the display device. The plastic elements are located between the glass optical wedge and the glass positive-power lens module, which protects them from scratching and other damage.
The use of the diffractive optical element achieves the correction of chromatic aberration in a highly efficient manner that allows weight reduction and also improves the color bandwidth of the display device. It is particularly effective in reducing higher-order chromatic aberration, which is otherwise very difficult to deal with for the large-pupil display device. Absent such a correction of the higher-order chromatic aberration, the pilot will see a rainbow effect upon rolling the eyes to view the off-axis projected image.
Thus, in one form, a display device for a display wavelength range comprises an image source, and a relay group made of optical elements transparent to the display wavelength range. The relay group comprises a glass optical wedge in facing relation to the image source, a glass lens, and a group of plastic lenses including a diffractive optical element, with the group of plastic lenses being positioned between the glass optical wedge and the glass lens. A reflective combiner is in facing relation to the glass lens.
The optical relay group has fewer optical elements than the relay lens group of the ""139 patent, and the optical elements of the present optical relay group are arranged in a single grouping. The present optical relay group is therefore compact in size. The use of some plastic optical elements reduces the weight of the optical relay group. The present optical relay group is therefore more compact and lighter in weight than prior comparable optical systems, leading to a low moment of inertia and less fatigue for the pilot. The plastic aspheric lenses may be either molded or diamond machined, at a relatively low cost, leading to a relatively low cost for the entire optical relay group.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.